The invention relates to a method of coding and decoding messages in which clear message pulses are mixed with code pulses at a transmitting station to form a coded message and in which the coded message pulses are mixed with identical code pulses at a receiving station to obtain the clear message pulses, the code pulses in the transmitting station and the receiving station being generated by a code generator the initial state of which determines the code pulse sequence, there being numerous possible initial states, and the initial state being determined from a first state information, designated the basic key, at the transmitting and receiving stations, and from a second state information, designated the message key, determined at random at one of the stations and transmitted to the other station.
Such a method is disclosed in U.S. Pat. No. 3,291,908.
In telecommunication networks in which, for reasons of secrecy of the messages, the above-mentioned method is used, there is often the need for a plurality of basic keys, for example, to render it possible to divide the stations into groups each having their own basic key or to classify messages into groups each having their own basic key. The number of basic keys which, for obvious reasons may alternatively be designated day keys, may amount to some dozens. The day keys are usually stored in protected storage compartments of the coding equipment: the designation key compartmentation was introduced.
The use of a plurality of day keys requires in a receiving station the setting of the proper day key, which may depend on the opposite station and/or the nature of the message. So far this setting has been done manually, which requires an action on the part of an operator, this manual setting also including the risk of mistakes.